The concrete panel is not loaded at all (other than by its own weight) when a train passes through the crossing. The loading of the panel occurs due to trucks and other vehicular road traffic passing over the crossing.
When a heavy truck passes over the crossing, the panel is subjected to bending stresses, in that the panel tends to deflect downwards between those points where the panel touches the cross-ties. In a case where the cross-ties are uneven, the panel might bridge over several cross-ties without actually touching. That is to say, the underside of the panel rests on the high-standing cross-ties, but is clear of the low-standing cross-ties.
If the panel is flexible enough, under a heavy road-traffic load, the panel might deflect so far that the undersurface of the panel touches the tops of the low-standing intermediate cross-ties. Once the panel touches the tops of the low-standing cross-ties, the panel is now supported by that cross-tie, and no further downwards deflection of the panel takes place.
The conditions that lead to premature failure occur when the cross-ties are unusually uneven. Given that all the cross-ties are mounted at exactly the same heights at their rail-attachment points, it might be considered surprising that the top surfaces of the cross-ties are not all at exactly the same heights, i.e that the top surfaces of the ties do not all lie in exactly the same flat, horizontal plane. However, there are a number of reasons for the unevenness. First, concrete cross-ties are moulded, and usually come from several different moulds, and the mould-maker would not have paid particular attention to getting all the moulds exactly equal. Also, the (moulded) concrete panel itself is large, and heavy, and its undersurface might not be completely flat. Also, some ties have writing embossed on the top surfaces. Concrete panels and concrete ties have metal reinforcing bars 1st into the concrete, and the bars can give rise to a slight distortion of the concrete components.
Naturally, the designer of the system takes account of the maximum unevenness of the tops of the cross-ties, and sees to it that the amount of stress the panel might undergo, in bending, will not cause the panel to fail. However, the panels still do seem to fail, and the notion has arisen that there must be some unknown factor affecting failure of the panels. Concrete panels are disfavoured by many railroad companies for this reason.
This is a pity, because concrete panels have the benefit that they can be installed quickly. One of the factors when working at road-rail crossings is that the crossing has to be closed--certainly to road traffic if not to rail traffic--for the period while the work is being done. One-piece panels offer the possibility that the panel can be pre-manufactured and brought to the site, and then the panel is simply hoisted up and lowered into position between the rails. The commercially-practical alternative to the one-piece concrete panel is to apply asphalt between the rails; however, where the one-piece panel takes just minutes to install, a corresponding asphalt installation takes hours. If only the concrete panels could find favour, on-site work could be carried out more quickly, and with pre-manufactured components, which would keep the on-site labour costs (and unpredictabilities) to a minimum.
The invention is aimed at providing a system for capping the cross-ties, in order to alleviate the problems of premature failure of the concrete panels.